Project Summary Impaired decision making is observed in almost all mental disorders. Many types of decisions rely on the gradual accumulation of evidence for the alternatives under consideration. In order to understand the neural circuit mechanisms that underlie evidence accumulation and develop targeted treatments when this process is impaired, we must first identify the neural circuit elements involved. An extensive body of research in primates and rodents has posited that the posterior parietal cortex (PPC) plays an important role in this process, but recent work has called into question the nature of this role. The long-term goal of this research is to first clarify the specific role of PPC for evidence accumulation as a foundation to understand the neural mechanisms that control the natural adaptability of decision policies. The proposed experiments will use a rat model system to examine the primary hypotheses for PPC's role that are consistent with the existing literature. The first aim will test whether PPC plays a direct but non-obligatory role in accumulation-based perceptual decision making. This will be made possible with the use of high temporal resolution optogenetic manipulations to perturb neural activity in PPC during evidence accumulation on randomly interleaved trials. The second aim will test whether PPC plays a role in decision commitment during perceptual decision making rather than a direct role in evidence accumulation. This will be achieved using a free response (reaction time) perceptual decision task where rats control the timing of decision commitment. This work is significant for elucidating the role of PPC, a brain region thought to be critical in decision making. The approach is innovative because it develops a conceptual framework for understanding decision processes at a single trial level in situations where subjects control when they respond in combination with optogenetic techniques in rodentsp to answer a previously inaccessible question. In the long term, we expect this research to harness both primate and rodent model systems to produce a detailed understanding of the neural circuit mechanisms that underlie evidence accumulation, decision commitment, and flexible control of both in decision making. The objective is to develop principled treatments targeted to specific neural circuits for the impairments to decision making associated with mental disorders.